package task;

import java.math.BigDecimal;

public class Pi implements Task {

	private static final long serialVersionUID = 1L;
	private static final int roundingMode = BigDecimal.ROUND_HALF_EVEN;
	private static final BigDecimal FOUR = BigDecimal.valueOf(4);
	private int digits;
    
    public void init(String init_str) {
    	this.digits = Integer.parseInt(init_str);
    }
    
    public Object execute() {
    	int scale = digits + 5;
    	BigDecimal arctan1_5 = arctan(5, scale);
    	BigDecimal arctan1_239 = arctan(239, scale);
    	BigDecimal pi = arctan1_5.multiply(FOUR).subtract(arctan1_239).multiply(FOUR);
    	return pi.setScale(digits, BigDecimal.ROUND_HALF_UP);
    }
    
    public static BigDecimal arctan(int inverseX, int scale) {
    	BigDecimal result, numer, term;
    	BigDecimal invX = BigDecimal.valueOf(inverseX);
    	BigDecimal invX2 = BigDecimal.valueOf(inverseX * inverseX);
    	numer = BigDecimal.ONE.divide(invX, scale, roundingMode);
    	
    	result = numer;
    	int i = 1;
    	do {
            numer = 
                numer.divide(invX2, scale, roundingMode);
            int denom = 2 * i + 1;
            term = 
                numer.divide(BigDecimal.valueOf(denom),
                             scale, roundingMode);
            if ((i % 2) != 0) {
                result = result.subtract(term);
            } else {
                result = result.add(term);
            }
            i++;
        } while (term.compareTo(BigDecimal.ZERO) != 0);
        return result;
    }
}